The term “tap coupler” refers in general to a three-port passive microwave device used in microwave art to divide power in an input path into two output paths.
The split of the input signal between output ports can be of equal or unequal value. In equal division, the power at the each of the two output ports is equal to the half of the input power and the ratio of output power, termed as split ratio, R, is 1:1 or 1 where one of the output ports has been standardized to 1. In the case of an unequal divider, the input signal is split into two output ports, such that the R-value is greater than 1.
Known power couplers include Lange coupler, branch line coupler, directional couplers, split-tee coupler and the Wilkinson coupler among others. U.S. Pat. No. 4,254,386 for a Three Way Equal-Phase Combiner/Divider Network Adapted for External Isolation Resistors is illustrative of several of these types of couplers.
U.S. Pat. No. 5,889,444 for a Broadband Non-Directional Tap Coupler shows further advance of the art in expanding bandwidth and manufacturability of such devices using a split-tee, five sections per arm design.
The directional coupler structures include single section, multiple section and tapered designs, among others. A comprehensive summary of such structures is provided in M. A. R. Gunston, “Microwave Transmission Line Data”, Noble Publishing, 1997, ISBN 1-884932-57-6. Gunston describes coupled transmission lines with coupled conductors of circular as well as rectangular cross sections. J. A. G. Malherbe, “Microwave Transmission Line Couplers”, Artech House, 1988, ISBN 0-89006-300-1 describes couplers with tapered conductors.
The need for unequal power dividers arises from the increased requirement for broadband, distributed, high power transmission line systems. U.S. Pat. No. 5,889,444 for a Broadband Non-Directional Tap Coupler describes familiar distribution line systems as a coaxial cable network, which brings cable signals to multiple TV sets. In more sophisticated systems, such as in-building bi-directional passive antenna distribution systems, transmitting and receiving signals of many services operating at different frequencies is desired. In this application, a main coaxial cable carries signals from a base station or off-air repeater located in the building to be distributed throughout the building. To add the branch lines to the main coaxial cable, and provide signal to the different floors, non-directional tap couplers are required. If each floor is to receive the same amount of power, the split ratio of the tap coupler should be adjusted at each floor. The split ratio used is between 3 dB (R of 1:1) and 30 dB (R of 1:999) for a typical in-building installation application.
Some difficulties associated with providing non-directional couplers for such an application are as follows. First, it is difficult to provide a tap coupler that can maintain a constant coupling value over a broadband frequency range. Second, it is difficult to provide a non-directional tap coupler where the input port voltage standing wave ratio (VSWR) is kept low or close to the minimum value of 1. VSWR is the ratio of the maximum voltage to minimum voltage on the transmission line. As known, when the load, such as the input of the tap-coupler, matches the characteristic impedance of the line, such as coaxial cable used to feed signal into tap coupler, there is no reflected wave present, so Vmax=Vmin and VSWR is 1. It is important to keep VSWR low to keep losses associated with the mismatch of the device to a minimum. Further, it is very difficult to provide a non-directional tap coupler that can handle high power and have low Passive Intermodulation Distortion Products (PIM). Furthermore, it is difficult to provide a non-directional tap coupler working over a wideband frequency range that provides physically realizable characteristic transmission line impedance values.
The present invention overcomes the shortcomings of prior art with a single section, compact, high power coaxial structure with a practical characteristic impedance range and achieves very good PIM performance with a high power rating.